1. Field of the Invention
The present invention relates to a sheet structure for electrical insulation or flame resistance comprising a barrier ply having a mica-rich face and a mica-poor face and a reinforcing ply containing a saturable backing layer attached to the mica-poor face of the barrier ply.
2. Description of the Related Art
The use of a mica-based sheets and tapes as an electrical insulating and flame barrier material is well known in the art. Such sheets and tapes typically consist of mica paper glued to a backing of reinforcing material.
U.S. Pat. Nos. 4,286,010, 4,769,276, 6,103,382, 6,153,301, and European Patent Application 0 373 137 A2, disclose the preparation and use of mica sheets and tapes in electrical insulation and flame protection applications. However, mica tapes produced by gluing mica paper to a backing reinforcement have limitations because the glue must be compatible with saturating resins that are commonly applied to the tapes in a subsequent step. The glue must also be thermally stable for extended periods of time. For example, a widely-used combination of backing glass cloth and epoxy resin as a glue limits the end-use temperature of the insulating tape to 180° C., as well as to the possible resins that may be used in the impregnation step (one normally must use the same epoxy resin). Also, the process of gluing of many resins usually involves continuous solvent-based processes and all the technological and safety complications related to dealing with volatile organics (ventilation, recuperation of the solvent and etc.).
An aramid-mica blended paper, made from a homogeneous blend of m-aramid fiber, m-aramid fibrids, and mica is manufactured by DuPont under trade name NOMEX® paper type 418. Such papers are described, for example, in U.S. Pat. No. 6,312,561. Because of superior mechanical properties vs. mica paper, tapes from NOMEX® 418 paper can be used without any backing reinforcement. However, 30-60 weight percent of organic (aramid) components in this barrier material limit its possible corona resistance or voltage endurance. Another limitation is that during vacuum-pressure impregnation of an item insulated with a tape based on this paper, it is necessary to rotate this item until the resin is completely cured because the tape is not readily saturable, that is the tape has no component that can hold uncured resin.
A two-ply paper has also been disclosed in which one ply consists of mica and another ply can consists of aramid, glass, polyester or other fibrous components (Russian Pat. 2051434, 1996). These papers do not have the necessary reinforcement and are relatively weak in sheet form and do not make acceptable narrow-width tapes.
What is needed is a sheet structure containing a high concentration of mica that can be made into tapes and easily saturated with a resin and that has sufficient mechanical properties for use in electrical insulation and flame retardant structures.